|MadSci Network: Physics|
My guess is the following: Whenever you have a phase change in a material, it usually requires some kind of assistance to get started. A good example of this is superheating. If you heat a (clean, unscratched) cup of water in a microwave it is possible to heat it slightly above its boiling point. This is possible because there is no convenient low energy site (like a fault in a surface) to allow the water molecules to break away from the water structure. If you then add some nucleation sites (like a teaspoon of instant coffee) the apparently quiescent water boils, violently. WARNING: Be very careful if trying this at home, the reaction is quite violent. My theory is that something similar happens when freezing water that is clean enough (no nucleation sites). If you give a sharp blow to subcooled water (slightly below its freezing point), this creates local pressure gradients (sound) which may act as inhomogeneities to let the ice crystals start to form. An interesting but I think unrelated phenomenon can be seen by cooling a bottle of beer down to a few degrees below zero. Because the beer is under pressure, the contents of the bottle are liquid. As soon as you open the bottle, however, the pressure drops and part of the beer freezes before your very eyes. It is quite fascinating to watch the ice crystals grow. (This will only occur with substances like water in which the solid is less dense than the liquid, for most substances the solid is more dense than the liquid).
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